Atomic structure of SrTiO3 (100) upon surface reduction

Due to concerns over problems with sample
conductivity, the first atomic resolution STM images on oxides were obtained on severely
reduced crystals and therefore produced surface structures that differed from the
ideal termination of the lattice. The first oxide imaged was TiO2 (110) on which
the periodic surface structures were attributed to stabilization of phases at the
surface based on crystallography. Similarly, the first images of atomic scale
structure on SrTiO3 were obtained on significantly reduced (001)
surfaces, figure 8.Large area scans illustrate the row-like structure observed on polished
and annealed surfaces. The structure consists of corrugations of several
distinct periodicities, in this case 4 Å and 12 Å, but always multiples of 4
Å. Corrugation amplitudes vary between 0.5 Å and 3 Å and atomic detail is observed in some regions.
A small region imaged with high resolution emphasizes the structural detail of
the corrugations in a region containing 12 Å and 20 Å corrugations. The variations
in corrugation periodicity on this surface cannot be described in terms of missing
oxygen rows because it was found that not only had oxygen left the surface but the Sr:Ti
ratio changed. The structures were described in terms of surface stabilized Ruddleson
Popper phases, known to be stable in bulk perovskite, and occurring as coherent intergrowths
of lamellae of reduced Srn+1TinO3n+1. The structures
are related to perovskite as shown below. The corrugation periods for the structure in
the figure are those of Sr2TiO4 and Sr3Ti2O7.These trends in stoichiometry were confirmed by
AES, and Rutherford back scattering spectroscopy (RBS).